ABSTRACT We are sedentary for the majority of our waking hours (e.g., reclining, sitting still) and time spent in these behaviors have been independently associated with reduced energy expenditure, increased risk for weight gain, and chronic disease. The efforts of researchers and health care providers interested in sedentary behavior (SB) would be greatly enhanced by a valid and easy- to-administer instrument for population-based studies that can assess SBs in terms of their context, setting, purpose, type, and amount. Such an instrument would reveal more fully the impact of SBs on health, and would allow a measurement of changes in behaviors in response to interventions. Our overall goal is to develop an instrument of manageable length that provides a comprehensive profile of major SBs in the population, and that produces a heterogeneous measure of these behaviors. We propose a research program to be conducted in two populations (adolescents and adults) at risk for the adverse health consequences of sedentarism. First, we will describe carefully the most important SBs in terms of the prevalence and duration using available national data sources and information from a de novo descriptive study (N=200). We will use these descriptive studies to develop and refine in parallel three SB questionnaires: two age-specific instruments (adolescents, adults) and a common measure that may be suitable for both age-groups. We will determine the feasibility of the common measure using results from our descriptive studies, cognitive testing, and expert review. If it is feasible we will evaluate it in the second portion of our research. If not, we will evaluate the age-specific instruments. Second, we will conduct a reliability and validity study of the new SBQ(s) in a separate sample of adolescents (n=200) and adults (n=200) using both self-report and objective measures to establish criterion validity. In addition to evaluating the psychometric properties of the SBQs via factor analysis, we also will conduct exploratory analyses to identify a subset of items that could be used as a short-form of the instrument(s). Third, we will disseminate the new SBQs and their scoring algorithms for free via the web. We anticipate that the instruments developed in this research program will facilitate studies that attempt to describe where and why SBs occur, etiologic studies that examine the adverse effects of SBs on health, and clinical and intervention studies that seek to quantify changes in these behaviors in response to an intervention or health promotion program.